1. Field of the Invention
The present invention relates to a steering system for automotive vehicles, and more particularly to a four-wheel steering system including a front wheel steering mechanism operable in response to a steering effort applied thereto to steer a pair of dirigible front road wheels and a rear wheel steering mechanism arranged to steer a pair of dirigible rear road wheels in response to steerage of the front road wheels.
2. Description of the Prior Art
In Japanese Utility Model Early Publication No. 60-92669, there has been proposed such a four-wheel steering system as described above, wherein the rear wheel steering mechanism is controlled to retain the rear road wheels in their neutral positions when the turn angle of the steering wheel is less than a predetermined small angle and to steer the rear road wheels in an opposite direction relative to the front road wheels when the turn angle of the steering wheel is larger than the predetermined small angle. Under such control of the rear wheel steering mechanism, however, the rear wheel steering angle is steppedly increased at a constant changing rate in accordance with an increase of the turn angle of the steering wheel. It is, therefore, apparent that if the changing rate of the rear wheel steering angle relative to the front wheel steering angle was determined in a small value, the rear road wheels would not be steered at a desired angle even when the front road wheels are steered at a larger angle than the predetermined small angle. This deteriorates the smaller turning ability of the vehicle. If the changing rate of the rear wheel steering angle was determined in a large value to enhance the smaller turning ability of the vehicle, the rear road wheels would be steered in excess when the front road wheels are steered from their neutral positions. This causes a great change in behavior of the vehicle and deteriorates the turning ability of the vehicle.